Precipitation hardening stainless steel powder, compound, granulated powder, precipitation hardening stainless steel sintered body, and method for producing precipitation hardening stainless steel sintered body

ABSTRACT

A precipitation hardening stainless steel powder, containing Cr at a concentration A within a range of 15.00 mass % or more and 17.50 mass % or less, Si at a concentration B within a range of 0.30 mass % or more and 1.00 mass % or less, Nb at a concentration C within a range of 0.15 mass % or more and 0.45 mass % or less, Ni at a concentration D within a range of 3.00 mass % or more and 5.00 mass % or less, Mn at a concentration E within a range of 0.05 mass % or more and 1.00 mass % or less, and Cu at a concentration F within a range of 3.00 mass % or more and 5.00 mass % or less, wherein a value of δ defined by the following formula (1) is 10.0 mass % or more and 14.0 mass % or less. 
       δ=3(A+1.5B+0.5C)−2.8(D+0.5E+0.5F)−19.8  (1)

The present application is based on, and claims priority from JP Application Serial Number 2019-165252, filed on Sep. 11, 2019, and 2020-100714, filed on Jun. 10, 2020, the disclosure of which is hereby incorporated by reference herein in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a precipitation hardening stainless steel powder, a compound, a granulated powder, a precipitation hardening stainless steel sintered body, and a method for producing a precipitation hardening stainless steel sintered body.

2. Related Art

In a powder metallurgy method, a composition containing a metal powder and an organic binder is molded into a desired shape, and thereafter, the obtained molded body is degreased to obtain a degreased body, and further, the degreased body is fired, whereby a sintered body is produced. In such a process for producing a sintered body, a phenomenon of atomic diffusion occurs among particles of the metal powder, whereby the molded body is gradually densified, resulting in sintering.

In such a powder metallurgy method, when the molded body is degreased, the organic binder is thermally decomposed and removed by heating the molded body. When the organic binder remains in the molded body, the properties of the sintered body are deteriorated, and therefore, various studies on the method for removing the organic binder have been conducted.

For example, JP-A-4-247802 (Patent Document 1) discloses that a degreasing treatment is performed by heating a molded body including a metal material powder and a binder containing a polyoxymethylene resin in an acid-containing atmosphere. By performing the degreasing treatment in an acid-containing atmosphere in this manner, the acid decomposes the binder, and therefore, the binder can be efficiently removed. Therefore, the above-mentioned problem can be reduced.

In the method described in Patent Document 1, it is considered that most of the organic binder is removed in the degreasing treatment. However, apart of the organic binder remains in the molded body, and is removed concurrently with the progress of sintering of the metal material powder in the subsequent firing treatment. At that time, for example, when the particle diameter of the metal material powder to be used is small or the like, the progress of sintering tends to accelerate. That is, sintering starts at a lower temperature stage in some cases. In such a case, the organic binder may be confined in the molded body during the firing treatment. As a result, there is a concern that an increase in the carbon atom concentration in the sintered body is caused, and the mechanical properties of the sintered body are deteriorated.

SUMMARY

A precipitation hardening stainless steel powder according to an application example of the present disclosure contains:

Cr at a concentration A within a range of 15.00 mass % or more and 17.50 mass % or less;

Si at a concentration B within a range of 0.30 mass % or more and 1.00 mass % or less;

Nb at a concentration C within a range of 0.15 mass % or more and 0.45 mass % or less;

Ni at a concentration D within a range of 3.00 mass % or more and 5.00 mass % or less;

Mn at a concentration E within a range of 0.05 mass % or more and 1.00 mass % or less; and

Cu at a concentration F within a range of 3.00 mass % or more and 5.00 mass % or less, wherein

a value of δ defined by the following formula (1) is 10.0 mass % or more and 14.0 mass % or less.

δ=3(A+1.5B+0.5C)−2.8(D+0.5E+0.5F)−19.8  (1)

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE illustrates a process chart showing a method for producing a precipitation hardening stainless steel sintered body according to an embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, embodiments of a precipitation hardening stainless steel powder, a compound, a granulated powder, a precipitation hardening stainless steel sintered body, and a method for producing a precipitation hardening stainless steel sintered body according to the present disclosure will be described in detail.

1. Precipitation Hardening Stainless Steel Powder

First, a precipitation hardening stainless steel powder according to an embodiment will be described.

In a powder metallurgy technique, a sintered body having a desired shape can be obtained by molding a composition containing a metal powder and a binder into a desired shape, followed by a degreasing treatment and a firing treatment. According to such a powder metallurgy technique, a sintered body with a complicated and fine shape can be produced in a near-net shape, that is, a shape close to a final shape as compared with the other techniques.

The precipitation hardening stainless steel powder according to an embodiment is a powder constituted by a precipitation hardening stainless steel containing Cr, Si, Nb, Ni, Mn, and Cu. In such a powder, Cr is contained at a concentration A within a range of 15.00 mass % or more and 17.50 mass % or less, Si is contained at a concentration B within a range of 0.30 mass % or more and 1.00 mass % or less, Nb is contained at a concentration C within a range of 0.15 mass % or more and 0.45 mass % or less, Ni is contained at a concentration D within a range of 3.00 mass % or more and 5.00 mass % or less, Mn is contained at a concentration E within a range of 0.05 mass % or more and 1.00 mass % or less, and Cu is contained at a concentration F within a range of 3.00 mass % or more and 5.00 mass % or less. Further, in such a powder, a value of δ defined by the following formula (1) is 10.0 mass % or more and 14.0 mass % or less.

δ=3(A+1.5B+0.5C)−2.8(D+0.5E+0.5F)−19.8  (1)

According to such a precipitation hardening stainless steel powder, sinterability can be suppressed while maintaining excellent mechanical strength derived from the precipitation hardening stainless steel. Due to this, in the firing treatment, the temperature at which sintering starts can be increased. As a result, an organic binder remaining in a molded body can be more reliably removed, and an increase in the carbon atom concentration in a sintered body can be suppressed. Therefore, the precipitation hardening stainless steel powder according to the embodiment enables production of a sintered body having high mechanical strength.

Hereinafter, the alloy composition of the precipitation hardening stainless steel powder according to the embodiment will be described in further detail. In the following description, the precipitation hardening stainless steel powder is sometimes simply referred to as “metal powder”.

1.1 Cr

Cr (chromium) is an element which mainly imparts corrosion resistance to a sintered body to be produced. By using the metal powder containing Cr, the corrosion resistance is enhanced, so that a sintered body having good corrosion resistance is obtained.

The concentration A of Cr in the metal powder is set to 15.00 mass % or more and 17.50 mass % or less, but is set to preferably 15.20 mass % or more and 16.90 mass % or less, more preferably 15.50 mass % or more and 16.70 mass % or less. When the concentration A of Cr is less than the above lower limit, the corrosion resistance of the sintered body to be produced may be insufficient depending on the overall composition. On the other hand, when the concentration A of Cr exceeds the above upper limit, the sinterability is deteriorated depending on the overall composition, and therefore, it becomes difficult to increase the density of the sintered body, so that the corrosion resistance or the mechanical properties of the sintered body may be deteriorated.

Note that the mechanical properties of the sintered body refer to, for example, properties such as mechanical strength and hardness.

1.2 Si

Si (silicon) is an element which mainly imparts corrosion resistance and high mechanical properties to a sintered body to be produced. By using the metal powder containing Si, the corrosion resistance and the mechanical properties are enhanced, so that a sintered body having good corrosion resistance is obtained.

The concentration B of Si in the metal powder is set to 0.30 mass % or more and 1.00 mass % or less, but is set to preferably 0.35 mass % or more and 0.95 mass % or less, more preferably 0.40 mass % or more and 0.90 mass % or less. When the concentration B of Si is less than the above lower limit, the corrosion resistance, the surface properties, or the mechanical properties of the sintered body to be produced may be deteriorated depending on the overall composition. On the other hand, when the concentration B of Si exceeds the above upper limit, the balance of the composition is likely to be lost depending on the overall composition, and therefore, the corrosion resistance, the surface properties, or the mechanical properties of the sintered body to be produced may be deteriorated.

Note that the surface properties of the sintered body refer to, for example, properties such as specularity and smoothness.

1.3 Nb

Nb (niobium) is an element which enhances the mechanical properties of a sintered body to be produced by precipitating a precipitate in the sintered body.

The concentration C of Nb in the metal powder is set to 0.15 mass % or more and 0.45 mass % or less, but is set to preferably 0.20 mass % or more and 0.40 mass % or less, more preferably 0.25 mass % or more and 0.35 mass % or less. When the concentration C of Nb is less than the above lower limit, precipitation of a precipitate is restricted in the sintered body, and therefore, the mechanical properties of the sintered body may not be able to be sufficiently enhanced. On the other hand, when the concentration C of Nb exceeds the above upper limit, a precipitate is excessively precipitated, and the density of the sintered body is decreased, and also the mechanical properties of the sintered body are deteriorated instead.

1.4 Ni

Ni (nickel) is an element which mainly imparts corrosion resistance and heat resistance to a sintered body to be produced. By using the metal powder containing Ni, the corrosion resistance and the heat resistance are enhanced, so that a sintered body having good corrosion resistance and surface properties is obtained.

The concentration D of Ni in the metal powder is set to 3.00 mass % or more and 5.00 mass % or less, but is set to preferably 3.50 mass % or more and 4.70 mass % or less, more preferably 3.80 mass % or more and 4.50 mass % or less. When the concentration D of Ni is less than the above lower limit, the corrosion resistance or the surface properties of the sintered body to be produced may not be sufficiently enhanced depending on the overall composition. On the other hand, when the concentration D of Ni exceeds the above upper limit, the balance of the composition is likely to be lost depending on the overall composition, and therefore, the corrosion resistance or the surface properties of the sintered body to be produced may be deteriorated.

1.5 Mn

Mn (manganese) is an element which imparts corrosion resistance and high mechanical properties to a sintered body to be produced in the same manner as Si. By using the metal powder containing Mn, the corrosion resistance and the mechanical properties are enhanced, so that a sintered body having good corrosion resistance and mechanical properties is obtained.

The concentration E of Mn in the metal powder is not particularly limited, but is preferably 0.05 mass % or more and 1.00 mass % or less, more preferably 0.07 mass % or more and 0.50 mass % or less, further more preferably 0.10 mass % or more and 0.40 mass % or less. When the concentration E of Mn is less than the above lower limit, the corrosion resistance, the surface properties, or the mechanical properties of the sintered body to be produced may not be sufficiently enhanced depending on the overall composition. On the other hand, when the concentration E of Mn exceeds the above upper limit, the corrosion resistance, the surface properties, or the mechanical properties may be deteriorated instead.

1.6 Cu

Cu (copper) is an element which enhances the mechanical properties of a sintered body to be produced by precipitating an intermetallic compound in the sintered body.

The concentration F of Cu in the metal powder is set to 3.00 mass % or more and 5.00 mass % or less, but is set to preferably 3.10 mass % or more and 4.50 mass % or less, more preferably 3.20 mass % or more and 4.20 mass % or less. When the concentration F of Cu is less than the above lower limit, precipitation of an intermetallic compound is restricted in the sintered body, and therefore, the mechanical properties of the sintered body may not be able to be sufficiently enhanced. On the other hand, when the concentration F of Cu exceeds the above upper limit, an intermetallic compound is excessively precipitated, and the density of the sintered body is decreased, and also the mechanical properties of the sintered body are deteriorated instead.

1.7 Value of δ

In the precipitation hardening stainless steel powder according to this embodiment, the value of 0.3 defined by the following formula (1) is 10.0 mass % or more and 14.0 mass % or less.

δ=3(A+1.5B+0.5C)−2.8(D+0.5E+0.5F)−19.8  (1)

The value of δ defined by such a formula (1) enables suppression of the sinterability of the precipitation hardening stainless steel powder without impairing the mechanical properties of a sintered body to be produced using the precipitation hardening stainless steel powder. Specifically, the first term of the right side of the formula (1) is a term related to elements for mainly producing a ferrite, and the second term is a term related to elements for mainly producing an austenite. In the ferrite, the diffusion rate during sintering is higher than that in the austenite, and therefore, the ferrite contributes to the enhancement of the sinterability of the precipitation hardening stainless steel powder.

In view of this, in this embodiment, by optimizing the ratios of the concentrations of the elements for producing the ferrite to the concentrations of the elements for producing the austenite based on the formula (1), the sinterability is suppressed while maintaining excellent mechanical strength derived from the precipitation hardening stainless steel in the sintered body to be obtained. More specifically, by setting the value of δ defined by the formula (1) within the above range, the sinterability of the precipitation hardening stainless steel powder is suppressed while maintaining the mechanical strength of the sintered body, and the diffusion rate can be decreased as compared with the related art. According to this, when the molded body is subjected to a firing treatment, the temperature at which sintering of the metal powder starts can be further increased. As a result, the organic binder remaining in the molded body can be more reliably removed at a stage before the metal powder starts to sinter.

The removal of the organic binder refers to volatilization of the organic binder or a decomposition product thereof, volatilization of a reaction product resulting from a reaction of a carbon atom contained in the organic binder with an oxygen atom contained in the metal powder or an oxygen atom adsorbed to the metal powder, or the like.

Here, the organic binder contains an organic compound as a main material, and therefore contains a carbon atom. When the organic binder in the molded body cannot be sufficiently removed, there is a concern that carbon atoms remain in the sintered body more than in the related art to decrease the mechanical strength of the sintered body. On the other hand, in this embodiment, by suppressing the sinterability of the precipitation hardening stainless steel powder, the removal efficiency of the organic binder in the sintering treatment can be increased. According to this, the carbon atom concentration in the sintered body can be suppressed, and a sintered body having high mechanical strength can be produced.

The value of δ is set to 10.0 mass % or more and 14.0 mass % or less, but is set to preferably 10.5 mass % or more and 13.5 mass % or less, more preferably 11.0 mass % or more and 13.0 mass % or less. When the value of δ is less than the above lower limit, the concentrations of the elements for producing the austenite become high. In that case, although the sinterability of the precipitation hardening stainless steel powder is lowered, the sinterability tends to become too low, and the density of the sintered body is hardly increased. As a result, a decrease in the mechanical strength of the sintered body to be obtained is caused. On the other hand, when the value of δ exceeds the above upper limit, the concentrations of the elements for producing the ferrite become high. In that case, the sinterability of the precipitation hardening stainless steel powder becomes too high, and the carbon atom concentration in the sintered body to be obtained becomes high. As a result, a decrease in the mechanical strength of the sintered body is caused.

1.8 Fe

Fe (iron) is an element whose content ratio is the highest among the elements contained in the precipitation hardening stainless steel powder according to the embodiment, that is, a principal component and has a great influence on the properties of the sintered body to be produced. The content ratio of Fe is not particularly limited, but is preferably 50 mass % or more, more preferably 60 mass % or more.

1.9 Other Elements

The precipitation hardening stainless steel powder according to the embodiment may contain, other than the above-mentioned elements, at least one element of C, Mo, W, N, S, and P as needed.

C (carbon) is an element which causes solid solution hardening as an interstitial element or causes precipitation hardening by a precipitate containing C or another element in a sintered body to be produced. By using the metal powder containing C, a sintered body having high mechanical properties is obtained. On the other hand, when the concentration of C, that is, the carbon atom concentration is too high, the mechanical properties, for example, the hardness of the sintered body is lowered.

The concentration of C in the metal powder is set to 0.07 mass % or less, but is preferably set to 0.01 mass % or more and 0.05 mass % or less. When the concentration of C exceeds the above upper limit, the balance of the composition is likely to be lost depending on the overall composition, and therefore, the mechanical properties of the sintered body to be produced may be deteriorated.

Mo (molybdenum) is an element which strengthens the corrosion resistance of a sintered body to be produced.

The concentration of Mo in the metal powder is not particularly limited, but is preferably 1.00 mass % or less, more preferably 0.01 mass % or more and 0.50 mass % or less. By setting the concentration of Mo within the above range, the corrosion resistance of the sintered body to be produced can be further strengthened without causing a significant decrease in the density of the sintered body.

W (tungsten) is an element which strengthens the heat resistance of a sintered body to be produced.

The concentration of W in the metal powder is not particularly limited, but is preferably 1.00 mass % or less, more preferably 0.01 mass % or more and 0.50 mass % or less. By setting the concentration of W within the above range, the heat resistance of the sintered body to be produced can be further strengthened without causing a significant decrease in the density of the sintered body.

N (nitrogen) is an element which enhances the mechanical properties such as proof stress of a sintered body to be produced.

The concentration of N in the metal powder is not particularly limited, but is preferably 1.00 mass % or less, more preferably 0.001 mass % or more and 0.50 mass % or less, further more preferably 0.05 mass % or more and 0.30 mass % or less. By setting the concentration of N within the above range, the mechanical properties such as proof stress of the sintered body to be produced can be further strengthened without causing a significant decrease in the density of the sintered body.

When the metal powder to which N is added is produced, for example, a method in which a nitrided raw material is used, a method in which nitrogen gas is introduced into a molten metal, a method in which the produced metal powder is subjected to a nitriding treatment, or the like is used.

S (sulfur) is an element which enhances the machinability of a sintered body to be produced.

The concentration of S in the metal powder is not particularly limited, but is preferably 0.50 mass % or less, more preferably 0.001 mass % or more and 0.30 mass % or less. By setting the concentration of S within the above range, the machinability of the sintered body to be produced can be further strengthened without causing a significant decrease in the density of the sintered body.

P (phosphorus) is an element which causes solid solution hardening as an interstitial element or causes precipitation hardening by a precipitate obtained by combination with another element in a sintered body to be produced. By using the metal powder containing P, a sintered body having high mechanical properties is obtained.

The concentration of P in the metal powder is set to 0.50 mass % or less, but is set to preferably 0.001 mass % or more and 0.35 mass % or less, more preferably 0.005 mass % or more and 0.30 mass % or less. When the concentration of P is less than the above lower limit, the mechanical properties of the sintered body may not be able to be sufficiently enhanced depending on the overall composition even if P is added. On the other hand, when the concentration of P exceeds the above upper limit, the balance of the composition is likely to be lost depending on the overall composition, and therefore, the mechanical properties of the sintered body to be produced may be deteriorated.

O (oxygen) may also be intentionally added or inevitably contained, however, the concentration thereof is preferably 0.01 mass % or more and 0.70 mass % or less, more preferably 0.15 mass % or more and 0.60 mass % or less. By making the concentration of 0 in the metal powder fall within the above range, silicon oxide is precipitated on the surface of the particle of the metal powder, and therefore, oxidation of an element such as Mn or Cr can be suppressed. As a result, the corrosion resistance and the surface properties of the sintered body to be finally produced can be enhanced.

When the content ratio of 0 is less than the above lower limit, the precipitation amount of silicon oxide is decreased, and therefore, oxidation of an element such as Mn or Cr may proceed. In that case, the corrosion resistance, the surface properties, and the mechanical properties of the sintered body to be produced may be deteriorated. On the other hand, when the content ratio of 0 exceeds the above upper limit, not only silicon oxide, but also an oxide of Mn or Cr is produced at the time of producing the metal powder. Due to this, it becomes difficult to increase the density of the sintered body to be produced, and further, along with this, the corrosion resistance, the surface properties, and the mechanical properties may be deteriorated.

In addition, the concentration of 0 is further more preferably 0.20 mass % or more and 0.55 mass % or less, and particularly preferably 0.33 mass % or more and 0.53 mass % or less. By making the concentration of 0 in the metal powder fall within the above range, in addition to the above-mentioned effect, an oxygen atom in the metal powder can be reacted with a carbon atom derived from the organic binder. For example, when an oxygen atom in the metal powder is present as silicon oxide, by a reaction represented by the following formula (2), the oxygen atom and the carbon atom can be removed from the sintered body.

SiO₂+2C→Si+2CO↑  (2)

By such a reaction, a carbon atom derived from the organic binder is consumed using an oxygen atom, and therefore can be removed. Accordingly, by incorporating a predetermined amount of oxygen atoms, a carbon atom concentration derived from the organic binder remaining in the sintered body can be suppressed.

When the concentration of O is less than the above lower limit, the carbon atom may not be able to be sufficiently consumed in the sintered body. On the other hand, when the concentration of O exceeds the above upper limit, the balance of the composition is likely to be lost, and therefore, the mechanical strength or the corrosion resistance of the sintered body to be produced may be deteriorated.

To the precipitation hardening stainless steel powder, other than the above-mentioned elements, H, Be, B, Al, Co, As, Sn, Se, Zr, Y, Ti, Hf, Ta, Te, Pb, or the like may be added in order to enhance the properties of the sintered body. In that case, the concentration of such an element is not particularly limited, but is set to such a concentration that the above-mentioned properties of the sintered body are not inhibited, and the concentration of each element is preferably less than 0.1 mass %, and even the sum of the concentrations is preferably less than 0.2 mass %. These elements are sometimes inevitably incorporated.

The precipitation hardening stainless steel powder may contain inevitable impurities. Examples of the impurities include all elements other than the above-mentioned elements. The concentration of each of these impurities may be any value as long as it is less than the concentration of each of Fe, Cr, Si, Nb, Ni, Mn, and Cu. Further, in particular, the concentration of each of these impurities is preferably less than 0.03 mass %, and the sum of the concentrations of these impurities is preferably less than 0.30 mass %. These impurities do not inhibit the effect as described above as long as the concentrations thereof are within the above range, and therefore may be intentionally added to the powder.

The tap density of the precipitation hardening stainless steel powder is preferably 3.5 g/cm³ or more and 5.5 g/cm³ or less, more preferably 4.0 g/cm³ or more and 5.0 g/cm³ or less. According to such a precipitation hardening stainless steel powder, the interparticle filling property becomes particularly high when obtaining a molded body. Therefore, a particularly dense sintered body can be finally obtained.

The specific surface area of the precipitation hardening stainless steel powder is preferably 0.10 m²/g or more and 0.70 m²/g or less, more preferably 0.15 m²/g or more and 0.50 m²/g or less. According to such a precipitation hardening stainless steel powder, a surface activity (surface energy) is optimized so that moderate sinterability is obtained. Due to this, a sufficient sintering rate can be obtained while preventing the organic binder or a decomposition product thereof, a reaction product of a carbon atom, or the like from remaining in the sintered body. As a result, a dense sintered body can be obtained while reducing the carbon atom concentration.

1.10 Analysis Method

The compositional ratio of the precipitation hardening stainless steel powder according to the embodiment can be determined by, for example, Iron and steel—Atomic absorption spectrometric method specified in JIS G 1257:2000, Iron and steel—ICP atomic emission spectrometric method specified in JIS G 1258:2007, Iron and steel—Method for spark discharge atomic emission spectrometric analysis specified in JIS G 1253:2002, Iron and steel—Method for X-ray fluorescence spectrometric analysis specified in JIS G 1256:1997, gravimetric, titrimetric, and absorption spectrometric methods specified in JIS G 1211 to G 1237, or the like. Specifically, for example, an optical emission spectrometer for solids, model: SPECTROLAB, type: LAVMB08A manufactured by SPECTRO Analytical Instruments GmbH, which is a spark optical emission spectrometer, or an ICP device, model: CIROS-120 manufactured by Rigaku Corporation is exemplified.

Note that JIS G 1211 to G 1237 are as follows.

JIS G 1211:2011: Iron and steel—Methods for determination of carbon content

JIS G 1212:1997: Iron and steel—Methods for determination of silicon content

JIS G 1213:2001: Iron and steel—Methods for determination of manganese content

JIS G 1214:1998: Iron and steel—Methods for determination of phosphorus content

JIS G 1215:2010: Iron and steel—Methods for determination of sulfur content

JIS G 1216:1997: Iron and steel—Methods for determination of nickel content

JIS G 1217:2005: Iron and steel—Methods for determination of chromium content

JIS G 1218:1999: Iron and steel—Methods for determination of molybdenum content

JIS G 1219:1997: Iron and steel—Methods for determination of copper content

JIS G 1220:1994: Iron and steel—Methods for determination of tungsten content

JIS G 1221:1998: Iron and steel—Methods for determination of vanadium content

JIS G 1222:1999: Iron and steel—Methods for determination of cobalt content

JIS G 1223:1997: Iron and steel—Methods for determination of titanium content

JIS G 1224:2001: Iron and steel—Methods for determination of aluminum content

JIS G 1225:2006: Iron and steel—Methods for determination of arsenic content

JIS G 1226:1994: Iron and steel—Methods for determination of tin content

JIS G 1227:1999: Iron and steel—Methods for determination of boron content

JIS G 1228:2006: Iron and steel—Methods for determination of nitrogen content

JIS G 1229:1994: Steel—Methods for determination of lead content

JIS G 1232:1980: Methods for determination of zirconium in steel

JIS G 1233:1994: Steel—Method for determination of selenium content

JIS G 1234:1981: Methods for determination of tellurium in steel

JIS G 1235:1981: Methods for determination of antimony in iron and steel

JIS G 1236:1992: Method for determination of tantalum in steel

JIS G 1237:1997: Iron and steel—Methods for determination of niobium content

Further, when C (carbon) and S (sulfur) are determined, particularly, an infrared absorption method after combustion in a current of oxygen or after combustion in a high-frequency induction heating furnace specified in JIS G 1211 (2011) is also used. As a specific analyzer, a carbon-sulfur analyzer, CS-200 manufactured by LECO Corporation is exemplified.

Further, when N (nitrogen) and O (oxygen) are determined, particularly, a method for determination of nitrogen content in iron and steel specified in JIS G 1228:2006 and a method for determination of oxygen content in metallic materials specified in JIS Z 2613:2006 are also used. As a specific analyzer, an oxygen-nitrogen analyzer, TC-300/EF-300 manufactured by LECO Corporation is exemplified.

In addition, in the sintered body produced using the precipitation hardening stainless steel powder according to the embodiment, a martensite crystal structure can be precipitated by performing any of various heating treatments. The martensite crystal structure imparts high hardness to the sintered body. It can be determined whether or not the sintered body has a martensite crystal structure by, for example, X-ray diffractometry.

The average particle diameter of the precipitation hardening stainless steel powder is preferably 0.50 μm or more and 50.0 μm or less, more preferably 1.00 μm or more and 30.00 μm or less, further more preferably 2.00 μm or more and 10.00 μm or less. By using the precipitation hardening stainless steel powder having such a particle diameter, pores remaining in the sintered body are extremely reduced, and therefore, a sintered body having a high density and excellent mechanical properties can be produced.

When the average particle diameter of the precipitation hardening stainless steel powder is less than the above lower limit, the moldability is deteriorated when molding the shape which is difficult to mold, and therefore, the sintered density may be decreased. On the other hand, when the average particle diameter of the precipitation hardening stainless steel powder exceeds the above upper limit, a gap between particles become larger during molding, and therefore, the sintered density may be decreased.

The average particle diameter of the precipitation hardening stainless steel powder can be determined as a particle diameter when the cumulative amount from the small diameter side reaches 50% in a cumulative particle size distribution on a mass basis obtained by laser diffractometry.

The maximum particle diameter of the precipitation hardening stainless steel powder is not particularly limited as long as the average particle diameter is within the above range, but is preferably 200 μm or less, more preferably 150 μm or less. By making the maximum particle diameter of the precipitation hardening stainless steel powder fall within the above range, the particle size distribution of the precipitation hardening stainless steel powder can be made narrower, so that the density of the sintered body can be further increased.

The maximum particle diameter refers to a particle diameter when the cumulative amount from the small diameter side reaches 99.9% in a cumulative particle size distribution on a mass basis obtained by laser diffractometry.

When the minor axis of each particle of the precipitation hardening stainless steel powder is represented by S [μm] and the major axis thereof is represented by L [μm], the average of the aspect ratio defined by S/L is preferably about 0.4 or more and 1 or less, more preferably about 0.7 or more and 1 or less. The precipitation hardening stainless steel powder having such an aspect ratio has a shape relatively close to a spherical shape, and therefore, the filling factor when the powder is molded is increased. As a result, the density of the sintered body can be further increased.

The major axis is the maximum possible length of the particle in a projected image, and the minor axis is the maximum possible length thereof in the direction perpendicular to the major axis. Further, the average of the aspect ratio is an average of the aspect ratio measured for 100 or more particles.

2. Method for Producing Precipitation Hardening Stainless Steel Sintered Body

Next, a method for producing a precipitation hardening stainless steel sintered body according to an embodiment will be described.

The FIGURE illustrates a process chart showing the method for producing a precipitation hardening stainless steel sintered body according to the embodiment.

The method for producing a precipitation hardening stainless steel sintered body shown in the FIGURE includes a composition preparation step S1 of preparing a composition for producing a sintered body, a molding step S2 of molding the composition, a degreasing step S3 of subjecting the molded body to a degreasing treatment, and a firing step S4 of subjecting the degreased body to a firing treatment. Hereinafter, the respective steps will be sequentially described.

2.1 Composition Preparation Step S1

First, the precipitation hardening stainless steel powder and an organic binder are kneaded using a kneader, whereby a kneaded material, that is, a compound according to an embodiment is obtained. The kneaded material is a composition containing the precipitation hardening stainless steel powder described above and the organic binder. By using such a kneaded material, although the organic binder is used, a sintered body having high mechanical strength can be produced.

The precipitation hardening stainless steel powder is produced by, for example, any of a variety of powdering methods such as an atomization method such as a water atomization method, a gas atomization method, or a spinning water atomization method, a reducing method, a carbonyl method, and a pulverization method.

Among these, the precipitation hardening stainless steel powder is preferably a powder produced by an atomization method, more preferably a powder produced by a water atomization method or a spinning water atomization method. The atomization method is a method in which a metal melt is caused to collide with a liquid or a gas sprayed at a high speed to atomize the metal melt into a fine powder and also to cool the fine powder, whereby a metal powder is produced. By producing the precipitation hardening stainless steel powder through such an atomization method, an extremely fine powder can be efficiently produced. Further, the shape of the particle of the obtained powder is closer to a spherical shape by the action of surface tension. Due to this, a powder having a high filling factor when it is molded is obtained. That is, a powder capable of producing a sintered body having a high density can be obtained.

When a water atomization method is used as the atomization method, the pressure of water to be sprayed onto the metal melt is not particularly limited, but is set to preferably about 75 MPa or more and 120 MPa or less, more preferably about 90 MPa or more and 120 MPa or less.

The water temperature of the atomization water is also not particularly limited, but is preferably set to about 1° C. or higher and 20° C. or lower.

The atomization water is often sprayed in a cone shape such that it has a vertex on the falling path of the metal melt and the outer diameter gradually decreases downward. In that case, the vertex angle θ of the cone formed by the atomization water is preferably about 10° or more and 40° or less, more preferably about 15° or more and 35° or less. According to this, a precipitation hardening stainless steel powder having a composition as described above can be reliably produced.

Further, by using a water atomization method, particularly, a spinning water atomization method, the metal melt can be particularly quickly cooled. Due to this, a powder having high quality can be obtained in a wide alloy composition.

Further, the cooling rate when cooling the metal melt in the atomization method is preferably 1×10⁴° C./s or more, more preferably 1×10⁵° C./s or more. By the quick cooling in this manner, a homogeneous precipitation hardening stainless steel powder is obtained. As a result, a sintered body having high quality can be obtained.

The thus obtained precipitation hardening stainless steel powder may be classified as needed. Examples of the classification method include dry classification such as sieving classification, inertial classification, and centrifugal classification, and wet classification such as sedimentation classification.

As the organic binder, a resin that can be decomposed in a short time in a degreasing treatment or a firing treatment is used. Examples of such a resin include polyolefins such as polyethylene, polypropylene, and ethylene-vinyl acetate copolymers, acrylic resins such as polymethyl methacrylate and polybutyl methacrylate, styrenic resins such as polystyrene, polyesters such as polyvinyl chloride, polyvinylidene chloride, polyamide, polyethylene terephthalate, and polybutylene terephthalate, polyether, polyvinyl alcohol, polyvinylpyrrolidone, and copolymers thereof, and various types of waxes, paraffins, higher fatty acids, higher alcohols, higher fatty acid esters, and higher fatty acid amides. Among these, one type can be used or two or more types can be used by mixing.

The mixing ratio of the organic binder is preferably about 2 mass % or more and 20 mass % or less, more preferably about 5 mass % or more and 15 mass % or less with respect to the total amount of the kneaded material. By setting the mixing ratio of the organic binder within the above range, a molded body can be formed with good moldability, and also the density is increased, whereby the stability of the shape of the molded body and the like can be particularly enhanced. Further, according to this, a difference in size between the molded body and the degreased body, that is, a so-called shrinkage ratio is optimized, whereby a decrease in the dimensional accuracy of the sintered body to be finally obtained can be prevented. That is, a sintered body having a high density and high dimensional accuracy can be obtained.

In the kneaded material, a plasticizer may be added as needed. Examples of the plasticizer include phthalate esters, adipate esters, trimellitate esters, and sebacate esters, and among these, one type can be used or two or more types can be used by mixing.

Further, in the kneaded material, other than the precipitation hardening stainless steel powder, the organic binder, and the plasticizer, for example, any of a variety of additives such as a lubricant, an antioxidant, a degreasing accelerator, and a surfactant, or another metal powder, a ceramic powder, or the like can be added as needed.

The kneading conditions vary depending on the respective conditions such as the alloy composition and the particle diameter of the precipitation hardening stainless steel powder to be used, the composition of the organic binder, and the blending amount thereof. However, in one example, the kneading temperature can be set to about 50° C. or higher and 200° C. or lower, and the kneading time can be set to about 15 minutes or more and 210 minutes or less.

Further, the kneaded material is formed into a pellet as needed. The particle diameter of the pellet is set to, for example, about 1 mm or more and 15 mm or less.

Note that depending on a molding method described below, in place of the kneaded material, a granulated powder according to an embodiment may be used. The kneaded material, the granulated powder, and the like are examples of the composition to be subjected to a molding step described below.

Such a granulated powder is obtained by binding a plurality of metal particles to one another with an organic binder by subjecting the precipitation hardening stainless steel powder to a granulation treatment. That is, the granulated powder is a composition containing the precipitation hardening stainless steel powder and the organic binder described above. By using such a granulated powder, although the organic binder is used, a sintered body having high mechanical strength can be produced.

Examples of the organic binder to be used for producing the granulated powder include the above-mentioned organic binders.

The mixing ratio of the organic binder is preferably about 0.2 mass % or more and 10 mass % or less, more preferably about 0.3 mass % or more and 5.0 mass % or less with respect to the total amount of the granulated powder. By setting the mixing ratio of the organic binder within the above range, a granulated powder can be efficiently formed while preventing significantly large particles from being granulated or a large amount of metal particles which are not granulated from remaining. Further, since the moldability is improved, the stability of the shape of the molded body and the like can be particularly enhanced. Further, by setting the mixing ratio of the organic binder within the above range, a difference in size between the molded body and the degreased body, that is, a so-called shrinkage ratio is optimized, whereby a decrease in the dimensional accuracy of the sintered body to be finally obtained can be prevented.

Further, in the granulated powder, other than the precipitation hardening stainless steel powder, the organic binder, and the plasticizer, for example, any of a variety of additives such as a lubricant, an antioxidant, a degreasing accelerator, and a surfactant, or another metal powder, a ceramic powder, or the like may be added as needed.

Examples of the granulation treatment include a spray dry method, a tumbling granulation method, a fluidized bed granulation method, and a tumbling fluidized bed granulation method.

In the granulation treatment, a solvent which dissolves the binder is used as needed. Examples of the solvent include inorganic solvents such as water and carbon tetrachloride, and organic solvents such as ketone-based solvents, alcohol-based solvents, ether-based solvents, cellosolve-based solvents, aliphatic hydrocarbon-based solvents, aromatic hydrocarbon-based solvents, aromatic heterocyclic compound-based solvents, amide-based solvents, halogen compound-based solvents, ester-based solvents, amine-based solvents, nitrile-based solvents, nitro-based solvents, and aldehyde-based solvents, and one type or a mixture of two or more types selected from these solvents is used.

The average particle diameter of the granulated powder is not particularly limited, but is preferably about 10 μm or more and 200 μm or less, more preferably about 20 μm or more and 100 μm or less, further more preferably about 25 μm or more and 60 μm or less. The granulated powder having such a particle diameter has favorable flowability, and can more faithfully reflect the shape of a molding die.

The average particle diameter is determined as a particle diameter when the cumulative amount from the small diameter side reaches 50% in a cumulative particle size distribution on a mass basis obtained by laser diffractometry.

2.2 Molding Step S2

Subsequently, the kneaded material or the granulated powder is molded, whereby a molded body having the same shape as that of a target sintered body is produced.

Examples of the molding method include a powder compaction molding method, a metal powder injection molding method, and an extrusion molding method.

The molding conditions in the case of a powder compaction molding method among these methods are preferably such that the molding pressure is about 200 MPa or more and 1000 MPa or less although varying depending on the respective conditions such as the composition and the particle diameter of the precipitation hardening stainless steel powder to be used, the composition of the organic binder, and the blending amount thereof.

Further, the molding conditions in the case of a metal powder injection molding method are preferably such that the material temperature is about 80° C. or higher and 210° C. or lower, and the injection pressure is about 50 MPa or more and 500 MPa or less although varying depending on the respective conditions.

Further, the molding conditions in the case of an extrusion molding method are preferably such that the material temperature is about 80° C. or higher and 210° C. or lower, and the extrusion pressure is about 50 MPa or more and 500 MPa or less although varying depending on the respective conditions.

The shape and size of the molded body to be produced are determined in anticipation of the shrinkage of the molded body in a degreasing step and a firing step described below.

2.3 Degreasing Step S3

Subsequently, the thus obtained molded body is subjected to a degreasing treatment, whereby a degreased body is obtained. Specifically, the degreasing treatment is carried out by decomposing and removing the organic binder.

Examples of the degreasing treatment include a method of heating the molded body and a method of exposing the molded body to a gas capable of decomposing the organic binder.

When the method of heating the molded body is used, the heating conditions for the molded body are preferably such that the temperature is about 100° C. or higher and 750° C. or lower and the time is about 0.1 hours or more and 20 hours or less, and more preferably such that the temperature is about 150° C. or higher and 600° C. or lower and the time is about 0.5 hours or more and 15 hours or less although slightly varying depending on the composition or the blending amount of the organic binder. According to this, the degreasing of the molded body can be necessarily and sufficiently performed without sintering the molded body. As a result, it is possible to prevent a large amount of the organic binder component from remaining inside the degreased body.

The atmosphere when the molded body is heated is not particularly limited, and examples thereof include an atmosphere of a reducing gas such as hydrogen, an atmosphere of an inert gas such as nitrogen or argon, an atmosphere of an oxidizing gas such as air, and a reduced pressure atmosphere obtained by reducing the pressure of such an atmosphere.

On the other hand, as the method of exposing the molded body to a gas capable of decomposing the organic binder, for example, an acid degreasing method is used. The acid degreasing method is a degreasing method utilizing the catalytic action of an acid by heating the molded body in an acid-containing atmosphere. By using the acid degreasing method, the organic binder can be decomposed in a short time even at a low temperature, and therefore, even the molded body having a large volume can be efficiently subjected to the degreasing treatment.

The acid-containing atmosphere refers to an atmosphere containing an acid capable of decomposing the organic binder. As the acid, for example, nitric acid, oxalic acid, ozone, and the like are exemplified, and among these, one type or two or more types in combination can be used. Further, a mixed gas obtained by mixing such an acid with another gas may be used. As an example of the mixed gas, fuming nitric acid is exemplified. The ambient pressure may be an atmospheric pressure, a reduced pressure, or an increased pressure.

The heating conditions for the molded body are preferably such that the temperature is about 100° C. or higher and 750° C. or lower and the time is about 0.1 hours or more and 20 hours or less, and more preferably such that the temperature is about 150° C. or higher and 600° C. or lower and the time is about 0.5 hours or more and 15 hours or less although slightly varying depending on the composition or the blending amount of the organic binder or the type of the acid-containing atmosphere. According to this, the degreasing of the molded body can be performed in a short time even at a relatively low temperature. In addition, the molded body can be prevented from being sintered or oxidized.

Note that by dividing such a degreasing step into a plurality of steps in which the degreasing conditions are different, and performing the plurality of steps, the binder in the molded body can be more rapidly decomposed and removed so that the binder does not remain in the molded body.

Further, according to need, the degreased body may be subjected to machining such as grinding, polishing, or cutting. The degreased body has relatively low hardness and relatively high plasticity, and therefore, machining can be easily performed while preventing the degreased body from losing its shape. According to such machining, a sintered body having high dimensional accuracy can be finally easily obtained.

2.4 Firing Step S4

Subsequently, the obtained degreased body is subjected to a firing treatment. By the firing treatment, surface diffusion occurs at the boundary surface between the particles of the precipitation hardening stainless steel powder, resulting in sintering. As a result, a sintered body is obtained.

The firing temperature varies depending on the composition, the particle diameter, or the like of the precipitation hardening stainless steel powder used in the production of the molded body and the degreased body, but is set to, for example, about 980° C. or higher and 1330° C. or lower, and is preferably set to about 1050° C. or higher and 1260° C. or lower.

Further, the firing time is set to 0.2 hours or more and 7 hours or less, and is preferably set to about 1 hour or more and 6 hours or less.

In the firing step, the firing temperature or the below-mentioned firing atmosphere may be changed in the middle of the step.

By setting the firing conditions within such a range, it is possible to sufficiently sinter the entire degreased body while preventing the sintering from excessively proceeding to cause oversintering and increase the size of the crystal structure. As a result, a sintered body having a high density and particularly excellent mechanical properties can be obtained.

Further, the thus produced sintered body may be subjected to an additional treatment as needed. Examples of the additional treatment include a solid solution treatment, an age hardening treatment, a double aging treatment, a sub-zero treatment, a tempering treatment, a hot working treatment, and a cold working treatment, and among these, one treatment or two or more treatments in combination are used.

Specific examples of the additional treatment include a treatment in which a solid solution treatment of cooling from a temperature of 1000° C. or higher and 1250° C. or lower over a time of 30 minutes or more and 120 minutes or less is performed, and thereafter, an age hardening treatment of cooling from a temperature of 600° C. or higher and 800° C. or lower over a time of 6 hours or more and 48 hours or less is performed.

3. Precipitation Hardening Stainless Steel Sintered Body

The precipitation hardening stainless steel sintered body according to this embodiment is a sintered body constituted by a precipitation hardening stainless steel containing Cr, Si, Nb, Ni, Mn, and Cu as described above. In such a sintered body, Cr is contained at a concentration A within a range of 15.00 mass % or more and 17.50 mass % or less, Si is contained at a concentration B within a range of 0.30 mass % or more and 1.00 mass % or less, Nb is contained at a concentration C within a range of 0.15 mass % or more and 0.45 mass % or less, Ni is contained at a concentration D within a range of 3.00 mass % or more and 5.00 mass % or less, Mn is contained at a concentration E within a range of 0.05 mass % or more and 1.00 mass % or less, and Cu is contained at a concentration F within a range of 3.00 mass % or more and 5.00 mass % or less. Further, in such a sintered body, a value of δ defined by the following formula (1) is 10.0 mass % or more and 14.0 mass % or less.

δ=3(A+1.5B+0.5C)−2.8(D+0.5E+0.5F)−19.8  (1)

According to such a precipitation hardening stainless steel sintered body, an increase in the carbon atom concentration is suppressed, and therefore, a sintered body having high mechanical strength derived from the precipitation hardening stainless steel is obtained. In particular, even when a sintered body is small or has a complicated shape, or the like, a carbon atom derived from the organic binder is prevented from remaining, so that a sintered body having high quality is obtained.

The precipitation hardening stainless steel sintered body can be used, for example, as a material constituting the whole or a part of a component for transport devices such as a component for automobiles, a component for bicycles, a component for railroad cars, a component for ships, a component for airplanes, or a component for space transport devices, a component for electronic devices such as a component for personal computers, a component for cellular phone terminals, a component for tablet terminals, or a component for wearable terminals, a component for electrical devices such as a refrigerator, a washing machine, and a cooling and heating machine, a component for machines such as a machine tool and a semiconductor production device, a component for plants such as an atomic power plant, a thermal power plant, a hydroelectric power plant, an oil refinery plant, and a chemical complex, or an ornament such as a component for timepieces, metallic tableware, jewels, and a frame for glasses.

As described above, the method for producing a precipitation hardening stainless steel sintered body according to this embodiment includes a molding step of molding the compound or the granulated powder containing the precipitation hardening stainless steel powder, thereby obtaining a molded body, and a firing step of firing the molded body, thereby obtaining a sintered body. In such a production method, the carbon atom concentration in the sintered body is preferably smaller than the carbon atom concentration in the precipitation hardening stainless steel powder. This is derived from the fact that the carbon atom concentration in the sintered body is decreased by the reaction of a carbon atom and an oxygen atom described above. By decreasing the carbon atom concentration in the sintered body in this manner, even when the precipitation hardening stainless steel powder having a high carbon atom concentration is used, the carbon atom concentration in the sintered body can be made to fall within the above range. According to this, a sintered body having high mechanical strength can be efficiently produced.

When the carbon atom concentration in the precipitation hardening stainless steel powder is represented by a first concentration c1 and the carbon atom concentration in the sintered body is represented by a second concentration c2, (c1−c2)/c1 is preferably 70 mass % or less, more preferably 50 mass % or less. According to this, the first concentration c1 can be reliably decreased, and therefore, the probability that the second concentration c2 falls within the above range becomes high. As a result, a sintered body having high mechanical strength, hardness, and corrosion resistance derived from the precipitation hardening stainless steel can be more reliably produced.

Hereinabove, the precipitation hardening stainless steel powder, the compound, the granulated powder, the precipitation hardening stainless steel sintered body, and the method for producing the same according to the present disclosure have been described with reference to preferred embodiments, however, the present disclosure is not limited thereto. For example, to the compound and the granulated powder, an arbitrary additive may be added.

The method for producing a precipitation hardening stainless steel sintered body according to the present disclosure may be a method to which a step for an arbitrary purpose is added to the above embodiment.

EXAMPLES

Next, Examples of the present disclosure will be described.

4. Production of Sintered Body Sample No. 1

[1] First, a precipitation hardening stainless steel powder having a composition shown in Table 1 produced by a water atomization method was prepared.

In the identification and quantitative determination of the composition of the metal powder shown in Table 1, inductively coupled high-frequency plasma optical emission spectrometry and an ICP device, model: CIROS-120 manufactured by Rigaku Corporation were used. Further, in the identification and quantitative determination of C, a carbon-sulfur analyzer CS-200 manufactured by LECO Corporation was used. Further, in the identification and quantitative determination of 0, an oxygen-nitrogen analyzer TC-300/EF-300 manufactured by LECO Corporation was used.

[2] Subsequently, the metal powder and an organic binder were weighed at a mass ratio of 89:11 and mixed with each other, whereby a mixed raw material was obtained. As the organic binder, a resin obtained by mixing a polyacetal resin containing butanediol at 2.5 mass % with polyethylene at a mass ratio of 50:6 was used.

[3] Subsequently, the mixed raw material was kneaded using a kneader, whereby a compound was obtained.

[4] Subsequently, the compound was molded using an injection molding machine under the following molding conditions, whereby a molded body was produced.

Molding Conditions

-   -   Material temperature: 180° C.     -   Injection pressure: 11 MPa (110 kgf/cm²)

[5] Subsequently, the obtained molded body was subjected to a degreasing treatment under the following degreasing conditions, whereby a degreased body was obtained.

Degreasing Conditions

-   -   Degreasing temperature: 400° C.     -   Degreasing time: 1 hour (retention time at the degreasing         temperature)     -   Degreasing atmosphere: a mixed gas atmosphere of nitrogen and         nitric acid, in which the concentration of nitric acid was 2 vol         %

[6] Subsequently, the obtained degreased body was subjected to a firing treatment under the following firing conditions, whereby a sintered body was obtained. The shape of the sintered body was determined to be a circular cylindrical shape with a diameter of 10 mm and a thickness of 5 mm.

Firing Conditions

-   -   Firing temperature: 1300° C.     -   Firing time: 3 hours (retention time at the firing temperature)     -   Firing atmosphere: argon atmosphere

[7] Subsequently, the obtained sintered body was sequentially subjected to a solid solution treatment and an age hardening treatment under the following conditions.

Conditions for Solid Solution Treatment

-   -   Heating temperature: 1120° C.     -   Heating time: 30 minutes     -   Cooling method: water cooling

Conditions for Age Hardening Treatment

-   -   Heating temperature: 700° C.     -   Heating time: 24 hours     -   Cooling method: water cooling

Sample Nos. 2 to 13

Sintered bodies were obtained in the same manner as in the case of Sample No. 1 except that the composition and the like of the precipitation hardening stainless steel powder were changed as shown in Table 1, respectively. Note that for the production of the powder of Sample No. 5, a gas atomization method was used.

In Table 1, among the precipitation hardening stainless steel powders and the sintered bodies of the respective Sample Nos., those corresponding to the present disclosure are denoted by “Ex.” (Example), and those not corresponding to the present disclosure are denoted by “Comp. Ex.” (Comparative Example).

Note that each sintered body contained very small amounts of impurities, but the description thereof in Table 1 is omitted.

5. Evaluation of Precipitation Hardening Stainless Steel Powder and Precipitation Hardening Stainless Steel Sintered Body 5.1 Measurement of Average Particle Diameter of Powder

With respect to the powders of the respective Sample Nos. shown in Table 1, the average particle diameter was measured. The measurement results are shown in Table 1.

5.2 Measurement of Tap Density of Powder

With respect to the powders of the respective Sample Nos. shown in Table 1, the tap density was measured. In the measurement of the tap density, a powder property evaluation device, Powder Tester (registered trademark) PT-X manufactured by Hosokawa Micron Corporation was used. The number of times of tapping was set to 125. The measurement results are shown in Table 1. Further, the powder of Sample No. 5 has a large particle diameter, and therefore, the measurement of the tap density was omitted.

5.3 Measurement of Specific Surface Area of Powder

With respect to the powders of the respective Sample Nos. shown in Table 1, the specific surface area was measured. In the measurement of the specific surface area, the BET method was used, and a BET specific surface area measuring device HM-1201-010 manufactured by Mountech Co., Ltd. was used. The amount of a sample was set to 5 g. The measurement results are shown in Table 1. Further, the powder of Sample No. 5 has a large particle diameter, and therefore, the measurement of the specific surface area was omitted.

5.4 Measurement of Carbon Atom Concentration in Sintered Body

With respect to the sintered bodies of the respective Sample Nos. shown in Table 1, the carbon atom concentration was measured. The measurement results are shown in Table 1.

5.5 Evaluation of Mechanical Strength of Sintered Body

A test piece specified in ISO 2740:2009 was cut out from each of the sintered bodies of the respective Sample Nos. shown in Table 1. Then, the tensile strength of the test piece was measured by the test method specified in JIS Z 2241:2011.

Subsequently, the tensile strength measured for the sintered body of Sample No. 10 was assumed to be 1, and the relative value of the tensile strength measured for each of the sintered bodies of the respective Examples and the respective Comparative Examples was calculated.

Then, the calculated relative value was evaluated according to the following evaluation criteria.

Evaluation Criteria for Tensile Strength

A: The tensile strength is very high (the relative value is more than 1.1).

B: The tensile strength is high (the relative value is more than 1 and 1.1 or less).

C: The tensile strength is low (the relative value is more than 0.9 and 1 or less).

D: The tensile strength is very low (the relative value is 0.9 or less).

The evaluation results are shown in Table 1.

5.6 Evaluation of Hardness of Sintered Body

With respect to the sintered bodies of the respective Sample Nos. shown in Table 1, the Vickers hardness was measured.

Subsequently, the Vickers hardness measured for the sintered body of Sample No. 10 was assumed to be 1, and the relative value of the Vickers hardness measured for each of the sintered bodies of the respective Examples and the respective Comparative Examples was calculated.

Then, the calculated relative value was evaluated according to the following evaluation criteria.

Evaluation Criteria for Hardness

A: The hardness is very high (the relative value is more than

B: The hardness is high (the relative value is more than 1 and 1.1 or less).

C: The hardness is low (the relative value is more than 0.9 and 1 or less).

D: The hardness is very low (the relative value is 0.9 or less).

The evaluation results are shown in Table 1.

5.7 Evaluation of Relative Density of Sintered Body

With respect to the sintered bodies of the respective Sample Nos. shown in Table 1, the density was measured by a method according to the Archimedes method. Then, the relative density of each sintered body was calculated based on the measured density and the true density of the precipitation hardening stainless steel powder.

Then, the calculated relative density was evaluated according to the following evaluation criteria.

Evaluation Criteria for Relative Density

A: The relative density is 98.0% or more.

B: The relative density is less than 98.0%.

The evaluation results are shown in Table 1.

5.8 Evaluation of Corrosion Resistance of Sintered Body

With respect to the sintered bodies of the respective Sample Nos. shown in Table 1, the corrosion rate was measured according to the method of sulfuric acid corrosion test for stainless steels specified in JIS G 0591:2012. As the sulfuric acid, boiled 5 mass % sulfuric acid was used.

Then, with respect to the corrosion rate measured for each of the sintered bodies of the respective Sample Nos., the relative value was calculated when the corrosion rate (unit: g/m²/h) measured for the sintered body of Sample No. 10 was assumed to be 1. Then, the calculated relative value was evaluated according to the following evaluation criteria.

Evaluation Criteria for Corrosion Resistance

A: The relative value of the corrosion rate of the sintered body is less than 0.75.

B: The relative value of the corrosion rate of the sintered body is 0.75 or more and less than 1.00.

C: The relative value of the corrosion rate of the sintered body is 1.00 or more and less than 1.25.

D: The relative value of the corrosion rate of the sintered body is 1.25 or more.

The evaluation results are shown in Table 1.

TABLE 1 Evaluation results of powder Average Alloy composition of precipitation hardening stainless steel powder particle Cr Si Nb Ni Mn Cu C O Fe δ diameter No. 1 Ex. 16.20 0.40 0.27 4.62 0.10 4.01 0.05 0.29 balance 12.32 7.11 No. 2 Ex. 16.53 0.36 0.31 4.54 0.16 4.06 0.05 0.33 balance 13.26 7.15 No. 3 Ex. 15.58 0.38 0.34 4.45 0.09 3.95 0.05 0.32 balance 11.04 6.89 No. 4 Ex. 15.25 0.40 0.25 4.38 0.11 3.87 0.05 0.32 balance 10.29 6.55 No. 5 Ex. 15.71 0.72 0.31 4.29 0.28 3.34 0.05 0.08 balance 13.96 15.31 No. 6 Ex. 15.10 0.78 0.39 4.56 0.35 3.85 0.05 0.66 balance 10.95 4.73 No. 7 Ex. 15.26 0.77 0.31 4.50 0.28 3.43 0.03 0.55 balance 12.12 4.78 No. 8 Comp. Ex. 15.54 0.37 0.28 4.75 0.22 4.15 0.05 0.25 balance 9.49 6.54 No. 9 Comp. Ex. 15.75 0.40 0.33 4.91 0.19 4.25 0.05 0.26 balance 9.78 7.52 No. 10 Comp. Ex. 16.91 0.55 0.25 3.46 0.15 3.98 0.05 0.29 balance 18.31 7.85 No. 11 Comp. Ex. 16.95 0.85 0.21 3.33 0.13 4.11 0.05 0.25 balance 19.93 6.78 No. 12 Comp. Ex. 17.10 0.83 0.35 3.43 0.12 3.43 0.05 0.28 balance 21.19 8.43 No. 13 Comp. Ex. 17.03 0.78 0.41 3.28 0.07 3.35 0.05 0.27 balance 21.44 6.91 Evaluation results of powder Specific Evaluation results of sintered body Tap surface Carbon atom Mechanical Relative Corrosion density area concentration strength Hardness density resistance No. 1 4.57 0.21 0.03 A A A A No. 2 4.38 0.19 0.04 A A A A No. 3 4.61 0.22 0.02 A A A A No. 4 4.73 0.24 0.05 B B A B No. 5 — — 0.07 B B B B No. 6 4.85 0.27 0.04 B B A B No. 7 4.80 0.26 0.01 A A A A No. 8 4.72 0.24 0.07 C C A C No. 9 4.48 0.21 0.07 C B A C No. 10 4.35 0.18 0.08 C C A C No. 11 4.44 0.19 0.11 C C A C No. 12 4.25 0.23 0.15 D D A D No. 13 4.59 0.18 0.17 D D A D

As apparent from Table 1, the sintered bodies of Examples had favorable mechanical strength, hardness, and corrosion resistance.

In addition, in the respective Examples, the carbon atom concentration in the sintered body was lowered as compared with the carbon atom concentration in the powder. Based on this, it is considered that in the respective Examples, the reaction product of a carbon atom is efficiently removed during the sintering treatment, and as a result, the mechanical properties and the corrosion resistance are improved.

Note that in the above description, the sintered body is obtained using a molded body produced by an injection molding method using a compound containing a precipitation hardening stainless steel powder. On the other hand, also with respect to a sintered body using a molded body produced by a compression molding method using a granulated powder containing a precipitation hardening stainless steel powder, the same evaluation as described above was performed. As a result, the same tendency as in the case of using the compound was observed. 

What is claimed is:
 1. A precipitation hardening stainless steel powder, comprising: Cr at a concentration A within a range of 15.00 mass % or more and 17.50 mass % or less; Si at a concentration B within a range of 0.30 mass % or more and 1.00 mass % or less; Nb at a concentration C within a range of 0.15 mass % or more and 0.45 mass % or less; Ni at a concentration D within a range of 3.00 mass % or more and 5.00 mass % or less; Mn at a concentration E within a range of 0.05 mass % or more and 1.00 mass % or less; and Cu at a concentration F within a range of 3.00 mass % or more and 5.00 mass % or less, wherein a value of δ defined by the following formula (1) is 10.0 mass % or more and 14.0 mass % or less: δ=3(A+1.5B+0.5C)−2.8(D+0.5E+0.5F)−19.8  (1)
 2. The precipitation hardening stainless steel powder according to claim 1, wherein O is contained at a concentration within a range of 0.01 mass % or more and 0.70 mass % or less.
 3. The precipitation hardening stainless steel powder according to claim 2, wherein O is contained at a concentration within a range of 0.33 mass % or more and 0.53 mass % or less.
 4. The precipitation hardening stainless steel powder according to claim 1, wherein an average particle diameter is 0.50 μm or more and 50.00 μm or less.
 5. A compound, comprising the precipitation hardening stainless steel powder according to claim 1 and an organic binder.
 6. A granulated powder, comprising the precipitation hardening stainless steel powder according to claim 1 and an organic binder.
 7. A precipitation hardening stainless steel sintered body, comprising: Cr at a concentration A within a range of 15.00 mass % or more and 17.50 mass % or less; Si at a concentration B within a range of 0.30 mass % or more and 1.00 mass % or less; Nb at a concentration C within a range of 0.15 mass % or more and 0.45 mass % or less; Ni at a concentration D within a range of 3.00 mass % or more and 5.00 mass % or less; Mn at a concentration E within a range of 0.05 mass % or more and 1.00 mass % or less; and Cu at a concentration F within a range of 3.00 mass % or more and 5.00 mass % or less, wherein a value of δ defined by the following formula (1) is 10.0 mass % or more and 14.0 mass % or less: δ=3(A+1.5B+0.5C)−2.8(D+0.5E+0.5F)−19.8  (1).
 8. A method for producing a precipitation hardening stainless steel sintered body, comprising: molding the compound according to claim 5 or the granulated powder according to claim 6, thereby obtaining a molded body; and firing the molded body, thereby obtaining a sintered body, wherein a carbon atom concentration in the sintered body is less than a carbon atom concentration in the precipitation hardening stainless steel powder. 